MEMS combine micro-scaled mechanical and electrical components into integrated systems. MEMS are typically used as microsensors, microactuators, and the like, and have found beneficial use for implementing accelerometers and other such inertial instruments. MEMS may also be used in chemical detectors, pressure sensors, thermal and/or electrostatic actuators, and the like. The use and applicability of such devices is only increasing as the intelligence and complexity of the MEMS increases, at the same time that the overall scale of the devices is decreasing into the nano-scaled, nanoelectromechanical systems (NEMS).
Many sub-millimeter MEMS/NEMS utilize capacitive connections or operations to implement the sensing or actuating functions. Moreover, many MEMS/NEMS use thermal energy for operation, which may require running electrical current across such MEMS/NEMS elements. The complexity of electronic circuitry for all types of these devices continues to increase. Therefore, in order to maintain the functionality of the capacitive elements, thermal elements, and the overall growing electronics, it is desirable to create MEMS/NEMS devices with electrical isolation properties. With the bulk of current technology settled mostly into the sub-millimeter MEMS region, techniques have been developed for fabricating micro-scaled devices with electrical isolation elements.
One such method, disclosed in U.S. Pat. No. 6,291,875, issued to Clark et al., entails etching a trench to physically separate the conductive material on the device and then filling that trench with an insulating material in order to re-attach the two portions. Thus, the electrical isolation is generally created by cutting the conductive connection and then mending the cut with an electrically isolating substance. With the insulating layer added, the device is again mechanically connected allowing the micromechanical aspect of the MEMS device to continue.
One problem associated with the trench-fill method for electrically isolating MEMS devices, are the cavities or voids that are typically formed in the insulating material filling the trench. The material used for the insulating layer typically does not uniformly fill the trenches. The unevenness may generally cause the upper portion of the trench to close before the lower portion of the trench is completely filled. This creates gaps or voids within the trench that can sometimes weaken the structural integrity of the device and can lessen the thermal conductivity, which is essential for reliable operation of some devices, such as thermal actuators.
The Clark, et al, patent discusses this problem and is directed to a method for improving the trench-fill by adding condyles to the trenches. Condyles are generally openings or “knuckles” at the trench ends that are wider than the basic trench width to allow the insulating material to more easily fill the trench more before closing off. Thus, the Clark patent requires etching trench patterns to attempt to alleviate the problems caused by the voids or cavities typically formed in regularly shaped trench-fills.
The addition of the condyles in the Clark patent does not guarantee that voids or cavities will not form. The increased opening areas likely improves the fill of the insulating material, but because of the non-uniformity and lack of precise control over the fill process, voids or cavities could still form for the same reasons.
Another method for implementing electrical isolation in sub-millimeter components is described in U.S. Pat. No. 6,239,473, issued to Adams, et al. Adams also describes an trench-fill method in its fabrication of MEMS beams with electrical isolation. Instead of attempting to overcome the problems caused by voids or cavities in the trench-fill, Adams specifically uses voids to form a fill layer that includes a re-entrant profile that increases the accuracy of the vertical etching necessary to form the Adams beams. Adams etches a teardrop shaped trench, with a smaller top portion and a larger bottom portion. This shape actually increases the tendency to form the void or cavity, and allows for the re-entrant profile of the trench-fill. When the step to etch the beam is executed, the re-entrant profile does not shield any of the silicon directly behind the protrusion of the trench-fill from being etched to form the beam. Therefore, Adams sacrifices some structural integrity and thermal conductivity caused by the voids in the trench-fill, to benefit from the re-entrant profile it can use to form its inventive beams. The Adams method, thus, suffers from the structural integrity and thermal conduction problems associated with trench-fill voids/cavities, in order to achieve electrical isolation for its specialized, highly-vertical beam structures.